Streaming long operations in NextJS - Mini Project: Build & Apply

Streaming Long Operations with Next.js

📖 Scenario: You are building a Next.js app that fetches a large list of user comments from a server. Instead of waiting for all comments to load, you want to show them as they arrive, improving user experience.

🎯 Goal: Create a Next.js component that streams comments from a server endpoint and displays them one by one as they arrive, using React Server Components and streaming techniques.

📋 What You'll Learn

Create a server API route that streams comment data in chunks

Create a React Server Component that fetches and renders comments as they stream

Use async iteration to process streamed data

Display each comment as soon as it arrives without waiting for all data

💡 Why This Matters

🌍 Real World

Streaming data is common in chat apps, live feeds, or large data loads where showing partial results quickly improves user experience.

💼 Career

Understanding streaming in Next.js and React Server Components is valuable for building fast, modern web apps that handle large or real-time data efficiently.

1

Create the comments data array

Create a constant array called comments with these exact string entries: 'Great post!', 'Thanks for sharing.', 'Very helpful.', 'Nice explanation.', 'Looking forward to more.'

NextJS

# Your code here

Hint

Use const comments = [ ... ]; to create the array with exact strings.

2

Create the streaming API route

Create an async function called GET that returns a Response streaming the comments array as JSON strings, one comment every 500 milliseconds. Use ReadableStream and setTimeout inside the stream controller to delay each chunk.

NextJS

const comments = ['Great post!', 'Thanks for sharing.', 'Very helpful.', 'Nice explanation.', 'Looking forward to more.'];

// Your code here

Hint

Use ReadableStream with a start(controller) method. Enqueue each comment as JSON string with a delay using setTimeout.

3

Create the React Server Component to stream comments

Create an async React Server Component called CommentsStream that fetches from /api/comments using fetch with cache: 'no-store'. Use for await to read the response body as a stream line by line, parse each JSON chunk, and render each comment inside a <li> element within a <ul>.

NextJS

const comments = ['Great post!', 'Thanks for sharing.', 'Very helpful.', 'Nice explanation.', 'Looking forward to more.'];

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      let i = 0;
      function push() {
        if (i >= comments.length) {
          controller.close();
          return;
        }
        const chunk = JSON.stringify({ comment: comments[i] }) + '\n';
        controller.enqueue(new TextEncoder().encode(chunk));
        i++;
        setTimeout(push, 500);
      }
      push();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'application/json' } });
}

// Create async React Server Component CommentsStream to fetch and stream comments
// Your code here

Hint

Use an async generator to read the stream line by line. Parse JSON and collect comments. Render them inside a <ul> with <li> for each comment.

4

Enable streaming rendering in the component

Modify the CommentsStream component to render comments as they arrive instead of waiting for all. Use React's use hook to await the async generator and render each comment inside a <li> as soon as it is received.

NextJS

const comments = ['Great post!', 'Thanks for sharing.', 'Very helpful.', 'Nice explanation.', 'Looking forward to more.'];

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      let i = 0;
      function push() {
        if (i >= comments.length) {
          controller.close();
          return;
        }
        const chunk = JSON.stringify({ comment: comments[i] }) + '\n';
        controller.enqueue(new TextEncoder().encode(chunk));
        i++;
        setTimeout(push, 500);
      }
      push();
    }
  });
  return new Response(stream, { headers: { 'Content-Type': 'application/json' } });
}

import React from 'react';

async function* streamComments() {
  const res = await fetch('/api/comments', { cache: 'no-store' });
  const reader = res.body?.getReader();
  const decoder = new TextDecoder();
  let buffer = '';

  while (true) {
    const { done, value } = await reader.read();
    if (done) break;
    buffer += decoder.decode(value, { stream: true });
    let lines = buffer.split('\n');
    buffer = lines.pop() || '';
    for (const line of lines) {
      if (line.trim()) {
        yield JSON.parse(line);
      }
    }
  }
  if (buffer.trim()) {
    yield JSON.parse(buffer);
  }
}

// Modify CommentsStream to render comments as they stream using React's use hook
// Your code here

Hint

Use React's use hook to await the async iterable converted to an array. Render comments inside <ul> and <li> elements.

Practice

1. What is the main benefit of using streaming in Next.js for long operations?

easy

A. Users see data progressively, reducing perceived wait time.

B. It automatically caches all data on the server.

C. It compresses data to reduce bandwidth usage.

D. It prevents any errors during data fetching.

Solution

1. Step 1: Understand streaming purpose

   Streaming sends data in chunks so users can see progress during long operations.

2. Step 2: Identify user experience impact

   This reduces the perceived wait time by showing partial results early.

3. Final Answer:

   Users see data progressively, reducing perceived wait time. -> Option A

4. Quick Check:

   Streaming improves user experience = A [OK]

Hint: Streaming shows partial data early to users [OK]

Common Mistakes:

• Thinking streaming caches data automatically
• Confusing streaming with data compression
• Assuming streaming prevents all errors

2. Which syntax correctly creates a streaming response in a Next.js API route?

easy

A. const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream);

B. const stream = new StreamReader(); stream.push('data'); return stream;

C. const stream = new WritableStream(); stream.write('data'); return stream;

D. const stream = new ReadableStream(); stream.send('data'); return stream;

Solution

1. Step 1: Recall correct ReadableStream usage

   ReadableStream requires a start method with a controller to enqueue data and close the stream.

2. Step 2: Match syntax with Next.js streaming pattern

   const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream); uses controller.enqueue and controller.close correctly and returns a Response wrapping the stream.

3. Final Answer:

   const stream = new ReadableStream({ start(controller) { controller.enqueue('data'); controller.close(); } }); return new Response(stream); -> Option A

4. Quick Check:

   ReadableStream with controller.enqueue = B [OK]

Hint: Use ReadableStream with start(controller) and enqueue [OK]

Common Mistakes:

• Using WritableStream instead of ReadableStream
• Calling non-existent methods like send() or push()
• Not closing the stream after enqueue

3. Given this Next.js API route code, what will the client receive?

export async function GET() {
  const stream = new ReadableStream({
    async start(controller) {
      controller.enqueue(new TextEncoder().encode('Hello '));
      await new Promise(r => setTimeout(r, 1000));
      controller.enqueue(new TextEncoder().encode('World!'));
      controller.close();
    }
  });
  return new Response(stream);
}

medium

A. Only "Hello " is received; stream never closes.

B. "Hello World!" received in two chunks with 1 second delay between.

C. "Hello World!" received all at once after 1 second delay.

D. Syntax error due to missing await in controller.enqueue.

Solution

1. Step 1: Analyze streaming behavior

   The stream first sends "Hello " immediately, then waits 1 second before sending "World!".

2. Step 2: Understand client reception

   The client receives data in two chunks with a delay, then the stream closes properly.

3. Final Answer:

   "Hello World!" received in two chunks with 1 second delay between. -> Option B

4. Quick Check:

   Streaming sends chunks with delay = C [OK]

Hint: Streaming sends chunks as they enqueue, not all at once [OK]

Common Mistakes:

• Assuming all data arrives after delay
• Thinking stream never closes
• Confusing async enqueue with syntax error

4. What is wrong with this Next.js streaming code snippet?

export async function GET() {
  const stream = new ReadableStream({
    start(controller) {
      controller.enqueue('Data chunk');
      // Missing controller.close()
    }
  });
  return new Response(stream);
}

medium

A. ReadableStream cannot be used in Next.js API routes.

B. controller.enqueue must be async function.

C. The stream never closes, causing the client to wait indefinitely.

D. Response must be wrapped in JSON.stringify.

Solution

1. Step 1: Check stream lifecycle

   The code enqueues data but does not call controller.close(), so the stream stays open.

2. Step 2: Understand client impact

   The client waits forever for the stream to end, causing a hang or timeout.

3. Final Answer:

   The stream never closes, causing the client to wait indefinitely. -> Option C

4. Quick Check:

   Missing controller.close() = stream hangs [OK]

Hint: Always call controller.close() after enqueueing all data [OK]

Common Mistakes:

• Forgetting to close the stream
• Thinking enqueue must be async
• Misunderstanding Response usage

5. You want to stream a large list of items from a Next.js API route, sending each item as JSON chunk. Which approach correctly streams the data without blocking the client?

hard

A. Send each item with separate fetch requests instead of streaming.

B. Convert entire list to JSON string, enqueue once, then close the stream.

C. Use WritableStream to write each JSON chunk and return it directly.

D. Use ReadableStream with async start(controller), loop over items, enqueue JSON strings, and close after all.

Solution

1. Step 1: Understand streaming large data

   Streaming each item as JSON chunk progressively sends data without waiting for all items.

2. Step 2: Identify correct streaming pattern

   Using ReadableStream with async start allows looping over items, enqueueing each chunk, then closing the stream.

3. Step 3: Evaluate other options

   A requires multiple requests (inefficient); B sends all data at once (no streaming benefit); C uses wrong WritableStream type.

4. Final Answer:

   Use ReadableStream with async start(controller), loop over items, enqueue JSON strings, and close after all. -> Option D

5. Quick Check:

   Stream chunks in loop with ReadableStream = D [OK]

Hint: Stream each item chunk inside async start, then close stream [OK]

Common Mistakes:

• Sending all data at once, losing streaming benefit
• Using WritableStream instead of ReadableStream
• Making multiple fetch calls instead of streaming